There is an increasing demand for real-time measurement of the structural integrity of critical moving components in modern gas turbines, which operate at ultra high temperatures. Moreover, requirements for gas turbines in the future call for ever higher operating temperatures. However, the harsh turbine environment encountered in these turbines along with the lack of long-term demonstrated sensor functionality, make it difficult to meet these objectives. The materials exposed to the hot gas path are being operated more closely to their design margins, which require the development of accurate and reliable sensors for monitoring the materials' condition during operation. While life prediction tools utilize laboratory generated data, active monitoring with embedded sensors enable a better understanding of the physics of material failure, loss of functionality due to material degradation, and coating delamination or cracking or spalling. The turbine typically includes a wide range of component materials with varied exposure to high temperatures and high centrifugal forces on rotating elements, and it is often surrounded by highly conductive metallic materials.
A temperature sensor with demonstrated reliability is a thermocouple. In principle, when two conductors made of dissimilar metals are joined at both ends, a thermoelectric circuit is formed. When one of the ends of this circuit is heated, there is a continuous current that flows in the thermoelectric circuit. Thomas Seebeck made this discovery in 1821. If this circuit is broken at the center, the hot open circuit voltage (the Seebeck voltage) is a function of the junction temperature and the composition of the two metals. Such a circuit is known as a thermocouple.
Most prior art thermocouples are made of wire. For example, a standard thermocouple, such as a Type S thermocouple, is made of a pure platinum wire for one wire leg and an alloy of approximately 90% platinum and 10% rhodium for the second wire leg. Another example of a standard prior art thermocouple is a Type R thermocouple, which has one wire leg made of an alloy consisting of 95% platinum and 5% rhodium, while the second wire leg is made of an alloy consisting of 87% platinum and 13% rhodium. These alloy combinations work well when the thermocouples are made of wire wherein the percent of the constituents along the length of the wire are consistent. This is not necessarily the case when the thermocouple legs are deposited in thin-film structures. For example, when an alloy (e.g., platinum-10% rhodium) is deposited, compositional variations along the length of the thermocouple leg will occur. That is to say, the 10% rhodium will not be evenly distributed everywhere, which will cause a deviation in the performance of the thermocouple.
Accordingly, there is a need for a thermocouple that is reliable and that can withstand the harsh environment of a gas turbine. While the thermocouple needs to function in a turbine hot gas path section where temperatures may be greater than 1300° C., it is also desired that the thermocouple has minimal impact on the performance or life of the component.